An Ultrafast Silicon Filter

A silicon wafer with 160 nanoporous silicon membranes. Each 15-nanometer-thick, 200-by-200-micrometers-square membrane is at the center of the 160 squares patterned into the wafer.

A porous silicon membrane that is a few nanometers thick can quickly filter liquids and separate molecules that are very close in size, researchers at the University of Rochester report in this week’s Nature. The new membrane could lead to efficient protein purification for use in research and drug discovery. It could also act roughly 10 times faster than current membranes used for blood dialysis, the artificial purification of blood. In addition, the membrane could be employed as a filter to separate molecules in microfluidics devices used to study DNA and proteins and as a substrate for growing neurological stem cells.

The polymer-based membranes currently used for filtering proteins are typically many micrometers thick and have an elaborate pore structure much like a sponge. “[Filtration] takes longer because there’s much longer distance to go through, and the pores are convoluted,” says Philippe Fauchet, the electrical- and computer-engineering professor at the University of Rochester who led the research. “And a fair fraction of what needs to go through remains stuck forever in the membrane.” Researchers end up losing the smaller protein molecules that are lodged inside, says James McGrath, a biomedical-engineering professor at Rochester and coauthor of the Nature paper.

The new membrane is 15 nanometers thick, so it filters faster without trapping the molecules that pass through it, which is important if researchers want to retain both the larger and smaller proteins. “Once a molecule gets to the membrane, it takes one step, and it’s on the back side,” McGrath says.

To make the membranes, the researchers employ tools that are used to create integrated circuit chips. This should make the filters easy to integrate into silicon-based microfluidic devices that are used for protein research, where they would be useful if scientists wanted to separate a particular protein of interest from a biological fluid sample. The researchers made the membranes by first depositing a stack of three thin layers–an amorphous silicon layer sandwiched between two silicon-dioxide layers–on a silicon wafer. Exposing the wafer to temperatures higher than 700 ºC crystallizes the amorphous silicon, and it forms pores. Then the researchers etch the wafer and silicon-dioxide layers to expose small squares of the nanoporous membrane that are 200 micrometers on each side. The temperature controls the pore diameter, allowing the researchers to fine-tune the membranes: at 715 ºC the membrane has an average pore size of 7 nanometers, while at 729 ºC the average is about 14 nanometers.